Jtratner datetime index insert non datetimes (fixes pandas GH5819)

This should fix https://github.com/pydata/pandas/pull/5819

allow concat in the indexes of a string to a datetime (converts to object; you may want to add a test
if you are concatting something like '20010101', e.g. a string that looks like a date)...I think it should NOT convert it though...fraught with too much risk of getting it wrong

that is why float/int are not allowed...because the datetimeindex is backed by ints...too much ambiguity here
rather raise than silently pass it thru 

the string case is oko thought I think

IIRC @cpcloud had this issue when concating a PeriodIndex and a DatetimeIndex..not sure what happened with that though